Inductively charged spraying apparatus

ABSTRACT

A spraying apparatus to deliver inductively charged spray, for example to crops. The apparatus includes a spray head and electrodes to apply a potential difference in the region of the spray head to charge spray. An aspirating means is arranged to remove liquid deposited on an electrode to reduce interference with the charging process. When the spray liquid is to be at high potential in the apparatus a long high resistivity supply tube may be provided. The aspirating means may be operated by spray liquid bled from a pressurized supply spray liquid.

The invention relates to inductively charged spraying apparatusparticularly for use in the spraying of growing crops.

Electrostatic spraying is known to be suitable for use in industrialapplications, such as paint spraying, where the conditions of operationcan be artificially controlled. Although conditions become much morevariable, the principle of applying an electrically charged spray isalso attractive for agricultural use since the surfaces of a growingplant are effectively maintained at earth potential and spray whichwould otherwise fall to the ground or drift away is likely to beintercepted. It is additionally advantageous in the use of pesticidesthat deposition on the underside of the leaves readily occurs.

The form of spray required will depend partly on the nature of the cropand its habit of growth and partly on the atmospheric conditions.Economy in the use of a concentrated formulation of spray liquid can beobtained by producing a mist of very small droplets. However such aspray must be used in suitable conditions of low air-movement and in theabsence of any significant velocity towards the ground has littleprobability of penetrating a tall or thick growth of foliage. For suchcrops it may be advantageous to use a pressurised spray which imparts asignificant velocity to the droplets in a chosen direction. Penetrationis then improved but generally the area covered is more limited.Processes of spray production are further distinguished according to themethods of atomising and of charging. Atomisation may be producedmechanically by disruption at a nozzle, as in the pressurised spray; orby disruption at the edge of a spinning disc; or electrically in thepresence of a strong field. The term "sprayhead" will be used to denoteatomisation mechanisms generally. Charging may be produced by theattachment of isolated charges from a corona discharge, in which casethe charged spray will usually be of the same polarity as the coronaelectrode and will be repelled from it. Corona charging however isuneconomic in the sense that little of the discharge current is usefullyapplied. An alternative method is to charge by induction either remotelyfrom earth or from a local induction electrode. In the latter case,which is preferred because lower potentials are involved, charge isproduced on the surface of the liquid just before atomisation by settingup a field in a relatively small gap between the local electrode and theliquid surface. The difficulty which arises in this mode of operation isthat, because charge separation occurs in the liquid, the charged sprayis of opposite polarity to the local electrode and the resultant forceof attraction causes a degree of surface wetting which is sufficient tointerfere with the charging process.

It is an object of the invention to provide improvements in inductivelycharged spraying apparatus.

In accordance with the invention there is provided an electrostaticspraying apparatus comprising a spray liquid supply means, a sprayheadhaving a spray liquid inlet and a spray exit, first and second electrodemeans supported in the apparatus to apply a potential difference in theregion of the spray exit whereby emergent spray is inductively chargedand aspirating means for at least one electrode to remove liquiddeposited on the electrode.

The first electrode may be associated with the sprayhead to be incontact with supplied spray liquid. The second electrode may be spacedfrom the spray exit and provided with the aspirating means.

In a preferred form the second electrode means comprises a hollow bodyhaving a wall portion and providing one or more apertures to allow thepassage to the interior of the body of liquid deposited on the wall.

The hollow body may be at least partly of conductive material when thespray liquid is of a non-conductive kind.

The aspirating means for such forms of second electrode means mayinclude a connecting tube between a source of reduced air pressure andthe interior of the body. The connecting tube may also providesupporting means for the second electrode means.

The source of reduced air pressure may include a liquid jet pump and thepump liquid may be provided by a reservoir which also supplies thesprayhead.

The aspirated liquid may be returned to the reservoir.

There is particular interest in apparatus in which atomisation occurs ata jet or jets as a result of a pressurised supply of liquid or liquidand air in combination, the size of the atomised particles having apredetermined relationship to the inlet pressure.

The sprayhead may then comprise a single jet producing a solid or hollowcone or fan of spray and the second electrode may be formed as acircular or elongate loop to preserve a substantially uniform spacingbetween the liquid surface and the nearest point on the electrode.

Alternatively the sprayhead may comprise a circular or linear array ofjets and the second electrode may be formed as a correspondinglycircular or elongate loop.

The sprayhead may be maintained at earth potential and the secondelectrode may be maintained at an elevated potential.

Alternatively the sprayhead may be at the elevated potential and thesecond electrode at earth potential.

The second electrode may then be arranged as a safety guard to preventcontact by the operator with the sprayhead and in that respect thealternative arrangement is preferred.

When the sprayhead is at elevated potential the liquid reservoir may beelectrically isolated from earth but it is preferred that it should havean electrical connection to earth and that the liquid is delivered tothe nozzle along a path arranged to provide a high electricalresistance.

In one form the path may comprise the bore of a long resistive walledtube. The length may be related to the resistivity of the liquid toprevent the leakage current from exceeding a predetermined value.

In another form where the electrode provided with aspirating means isfor use at a potential other than earth potential the aspiration flowpath means may be long to reduce electrical leakage from the electrodethrough the flow path means and any liquid therein.

The second electrode may include at least a portion formed of porousmaterial through which the aspirating means can act to remove depositedliquid. The porous material may be supported away from the spray exit bysupport means in the second electrode. The support means for the porousmaterial may act to collect liquid.

In a spraying arrangement for field use there may be at least onespraying apparatus as described above together with a supply meansincluding a spray liquid reservoir, spray liquid pump means to forceliquid from the reservoir to the spray head, aspiration means to providea sub-ambient pressure source for the aspirating means, power supplymeans to exert said potential difference, and support means to maintainthe at least one spraying apparatus in a spray delivering configurationfor directed delivery of charged spray liquid.

The spraying arrangement may be arranged for vehicle mounting or it maybe arranged for hand-held use.

An embodiment of the apparatus of the invention and the manner ofoperation will now be described with reference to the accompanyingdrawings in which:

FIG. 1 represents diagrammatically a sprayhead and electrode assembly inaccordance with the invention; and

FIG. 2 represents from below a diagrammatic view of alternativesprayhead and electrode members for the assembly of FIG. 1,

FIG. 3 represents diagrammatically another embodiment of the invention,and

FIG. 4 represents a perspective diagrammatic view of a spray head andelectrode members embodying the invention.

Referring now to FIG. 1 an assembly comprising a sprayhead 10 and anelectrode unit 12 is shown, partly in section, mounted vertically forthe downward spraying of a liquid of significant electricalconductivity. Sprayhead 10 comprises a hollow inlet column 14 ofinsulating material, closed at the upper end and having at the lower enda chamber 16 which carries a commercially available nozzle 18 of brassor similar material. A single hole 20 in the tip of nozzle 18 isarranged to produce a hollow conical spray pattern of about 80° apicalangle. Liquid to be sprayed is supplied from a reservoir 22 through atube 24 to an inlet position 26 near the upper end of inlet column 14.The liquid pressure required for the desired operating characteristicsof the nozzle is provide by a pump 28 connected into tube 24. Anelectrical input lead 30 is connected in the present embodiment betweenthe high voltage terminal of a supply unit 32 and a bushed terminal 34set in the wall of inlet column 14. A conductive portion of terminal 34which projects within column 14 makes contact with incoming liquid.

The electrode unit 12 comprises a tubular ring electrode 36 which issuspended by metallic support legs 38, 40 from a collar 42 on column 14.Electrode 36 is mounted coaxially with column 14 to lie in a plane whichis at the general level of the tip of nozzle 18 or slightly above orbelow that level. In the position represented in FIG. 1 electrode 36 isbelow the exit level of nozzle 18 by a distance of about 10 mm and thediameter of electrode 36 is about 30 mm. Electrode 36 includes a hollowbody providing one or more apertures to allow the passage, byaspiration, to the interior of the hollow body of liquid deposited onthe electrode. In order to minimise the electric intensity at thesurface of electrode 36 the electrode is formed from tubing of roundsection, about 5 mm in diameter. At or near the lowest periphery ofelectrode 36 the tubular wall is pierced by a succession of holes 44closely spaced round the circumference. The hole size is not criticalbut should be large enough to allow liquid to be drawn through the holesto the inside of the tube under reduced pressure but small enough toprevent liquid from readily falling through the hole from the inside. Adiameter of about 0.5 mm is suitable. Support leg 38 is a solid rod bentoutwards so that for its whole length it is at a greater spacing fromnozzle 18 than is electrode 36. Support leg 40 is similarly shaped butis made from tubing which is soldered into the wall of electrode 36 toprovide a suction passage. At the upper open end of leg 40 is connecteda length of tubing 46 which leads to a pump 48, the output from which isreturned to reservoir 22. Pump 48 is required to produce a pressurereduction of only a few millibars below atmospheric pressure and a jetpump, or ejector, having a flow control valve and operating on a supplyderived in a tube 49 from the main inlet flow in tube 24 can be used forthis purpose.

Collar 42 includes a wire ring or a spider 50 which carries legs 38, 40and is arranged to be electrically connected to earth either directly orthrough the framework of a mobile machine if the spraying unit ismounted for use in this way. Since some of the lighter spray particlesmay drift upwards from the nozzle to be deposited on the ring 50 and onadjacent surfaces the underside of collar 42 is provided with drainagegrooves terminating in a toothed edge 52 which serves to interceptdeposited liquid. Such liquid would otherwise run down column 14 to thenozzle to cause electrical leakage.

In operation, if only the liquid supply pump 28 is switched on, aconical pattern of uncharged spray is produced from nozzle 18. It is anadvantage of the pressurised spray that, although dependent on thepressure of the supply, drop production is substantially invariant withother conditions of operation. Thus the spray can continue to be usedeven though the high-voltage power supply may fail.

When supply unit 32 is switched on at say 5 kV the liquid reachingnozzle 18 is maintained at that potential and immediately outside thenozzle an electric field is created which corresponds to the potentialdifference of 5 kV between electrode 36 and the liquid at the nozzleexit (or the nozzle itself). By a flow of current through the sheet ofliquid which extends from the nozzle hole 20 in the preliminary stage ofatomisation a charge is induced on to the surface of the sheet and iscarried away by the resultant droplets. In the presence of electrode 36at earth potential the charged particles experience a force of outwarddeflection from the original conical path so that the apical angle ofthe cone is increased. A small proportion of spray particles aresufficiently deflected to be deposited on elctrode 36 where they readilyform cusped extensions to the electrode profile at which the electricfield is strong enough for a corona discharge to occur. The inductivecharging effect then either fails or is greatly reduced.

By operating the aspirator however, that is by causing pump 48 to reducethe pressure in tube 46 and thence within leg 40 and within electrode36, the deposited drops of liquid on the outer surface of electrode 36are drawn through the holes 44. In this way the electrode surface iskept free of any accumulation of liquid, although a film will alwaysremain, and the efficiency of the charging process is maintained. Sincethe aspirated liquid is returned to reservoir 22 there is no wastage ofliquid. If, as indicated in a previous paragraph, the reduction inpressure is induced in a jet pump by the main inlet pressure theaspirator action is automatically effective as soon as pump 28 isswitched on. It is desirable to recover the liquid not directed as sprayfor several reasons. The chemicals involved are usually expensive. Lossof liquid from the spray leads to erroneous dosage rates and eitherinsufficient spray application or an uncertain margin for the loss.Liquid drops falling on the plants can cause damage by the largequantity of chemical at one spot. Liquid drops on the soil or operatorcan be taken elsewhere and cause damage to other crops.

Nozzle 18 has been described as having a single circular hole 20 toproduce a hollow conical spray pattern. Different spray patterns may bedesirable for different crops, the simplest alternative being awedge-shaped or fan-shaped distribution for which a nozzle having asuitably formed orifice is commercially available. FIG. 2 shows frombelow only the relevant features of such a fan nozzle 60 and acorrespondingly elongate loop form of electrode 62. The loop is formedso that a substantially similar electric field is experienced by allparts of the fan-shaped liquid sheet. Electrode 62 is provided withaspirator holes 44 and the arrangement is generally similar to that ofFIG. 1. Electrodes may be designed on the same basis for nozzles havinga plurality of holes in circular or linear array.

The conductivity of the liquid must be taken into account in severalaspects of design of the apparatus. The first consideration is thedegree of conductivity which is necessary to enable the charging processto proceed. In the simplest picture a sheet or a thread of liquidextending from the nozzle and exposed to an electric field must providea leakage path to the supply circuit in order to retain a net charge onthe droplets formed as the sheet or thread breaks up.

A numerical estimate can be made as to the suitability of a liquid byconsidering that charge must flow within the time of exposure of theliquid to the field. For example, for a nozzle operated at a pressure ofthe order of 4 bars and almost independently of nozzle size the initialjet velocity is about 20 meters/sec. If the jet is unbroken over anaxial distance of (say) 10 mm the liquid will escape the influence ofthe field in about 0.5 ms. Provided the product (resistivity xpermittivity) is below this value effective charging will result. Forwater the product is equal to about 10 ns so that a factor of 10⁴ ormore relative to water is available to accommodate materials of higherresistivity.

The conditions at a metal nozzle may differ from the liquid threadconcept in enabling charging to occur in the field applied to the thinlayer of liquid which instantaneously is present at the surface of thenozzle. For liquids of relatively low resistivity it is likely that aninsulating material such as a plastics or ceramic material could be usednot only for column 14 but also advantageously for nozzle 18. The liquidwould then provide a continous conductive path and the field distortioncaused by the presence of the metallic surface would be avoided.

A further possibility is that electrode 36 is made from insulatingmaterial. A conducting film can be applied to the surface but if this isomitted the surface will receive a conductive layer of liquid as soon asspraying is started. By means of the connection provided by the supportlegs 38, 40 the wetted surface of electrode 36 is maintained at earthpotential. An advantage of such an arrangement is that electrode 36 canbe made readily and cheaply replaceable in case of blockage of theaspiration holes 44 or to provide a different form of aspirationaperture such as a slot. Thus in such an embodiment support leg 38 cancarry an attachment clip to fit the ring of electrode 36 at one end of adiameter and a plug-in tubular connection is formed in the wall ofelectrode 36 at the other end of the diameter to engage with tubularsupport leg 40.

In the consideration of optional variations in the materials ofconstruction of the nozzle and the electrode, the conductivity of theliquids commonly used for spraying can be used to advantage. The problemremains however that once liquid which remains within the sprayingsystem has been raised to an elevated voltage it must either be isolatedor returned to earth potential for reasons of operator safety. Anisolated reservoir, however, represents a substantial capacitance and isdangerous when it is charged to a potential of several kV. It ispreferred that the reservoir should be at earth potential provided thatthe leakage current from the HT connection at the nozzle can berestricted to an acceptable and predetermined level. It has been foundthat the delivery tube 24, if made from a strong, flexible and highlyinsulating material such as nylon, can have a small bore and an extendedlength such that leakage through the liquid is less than 10 μA per kV.The result for any chosen combination of length and bore is easilycalculated and the combination which is favoured will of course dependon the pressure required at the nozzle and the pressure available at thepump 28. The pressure drop along the tube for a given volumetric flowrate is very sensitive to the bore dimension (the inverse fifth power)and length is the preferred variable. A long tube can be accommodatedcompactly by coiling or by stacking straight lengths of the tube joinedfor continuity by means of U-pieces at each end. For the nozzle 18having a flow rate of 1.5 mL/s a pressure drop along the tube of lessthan 10% of the nozzle pressure is found for a water supply of 110 MΩresistance. For the same tube, to supply the nozzle 60 having a flowrate of 4.8 mL/s, the pressure drop is about 40% of the correspondinglylower nozzle pressure so that generally the available working pressureof the pump is not a limitation.

The leakage current for water, in this experiment, is still undesirablyhigh relative to the current carried by the charged spray and a furtherincrease in length or a small reduction in bore size, if commerciallyavailable, is indicated. For a liquid of higher resistivity there is nodifficulty in defining a suitable supply tube.

Experiments have been conducted which demonstrate the value, in terms ofcontrolled deposition, of inductive charging of the pressurised sprayproduced by the nozzles illustrated in FIGS. 1 and 2. For example bymeasuring the deposit on metal targets in a wind tunnel, for an airspeed of 2 m/s the deposition over the first meter of drift distance wasfound to be twenty times as great for the charged spray as for theuncharged spray. Measurements on the mean drop size and sizedistribution showed insignificant variation between the charged anduncharged state, thus confirming that the atomisation process consideredis predominantly mechanical. There is considerable difficulty inestablishing consistency of charge measurement between differentexperimenters in different situations and while it is thought thatcharge:mass ratios between 0.5 and 1.0 mC/kg (dependent on the nozzledesign) are representative of operation at 5 kV in these experiments,the response of the deposition pattern to the presence of charge isconsidered of greater significance.

Charge measurements have also been made which establish the value ofproviding for the continous removal of deposited liquid from theinduction electrode in accordance with the invention. Thus at 5 kV, avoltage which produces an effective but not very high field intensity inthe electrode-nozzle gap, an increase in charge:mass ratio of at least50% is observed. This improvement is assumed to be due to the preventionof the loss of charge from the deposited liquid which is otherwisecaused by an invisible corona discharge. At higher voltages in theabsence of aspirator operation the indicated charge rapidly becomesunstable. Cusps are formed on the surface of the deposited liquid andsparking can be observed between the cusps and the electrode 36.

It will be apparent that operation of the apparatus as described remainsidentical in principle if the polarities are reversed, the nozzle andthe liquid supply being then at earth potential. There is in fact thenno contribution to charging by induction from earth but the localinduction electrode is in any case assumed to be predominant. Theproblem of leakage from the HT supply through the liquid delivery tubeto earth is avoided but leakage from the induction electrode through theaspirator line must be taken into account and may be made acceptablysmall in a manner similar to that described. The presence of the highvoltage electrode and support assembly re-introduces a safety problemwhich can be solved by routine precautions but which is avoided by thepreferred high voltage nozzle embodiment.

FIG. 3 shows another embodiment of the invention in this case with theelectrode unit 112 arranged for use at a potential other than earth. Itis believed that in some applications there is an advantage in forcingthe spray particles past a high-potential electrode towards an earthedtarget such as a plant.

The spray head 110 in FIG. 3 is basically spray head 10 of FIG. 1 withmodifications to the electrical arrangement to use it as the earthelectrode. The body 114 can be of conductive material if preferred orthe spray liquid therein can be earthed via an electrode as describedabove for supply electrode 34. The liquid supply 124 does not have toprovide any insulation and the whole supply means can be earthed asindicated by the earth symbol on reservoir 22. The high potential frompower supply unit 32 is applied to electrode 136 which is insulatinglysupported in spaced relationship to nozzle 18 by supports such as 138,139, 140. Two, three, four or more supports may be used. In general thefewer, having regard to stability, the better, to reduce electricalleakage. Electrode 136 is typically of conductive material, such asmetal or plated plastics, but if the high voltage connection is extendedto region of spray liquid deposit and the liquid is conductive anelectrode of insulating material, as mentioned above, could be used.

Supports 138, 140 have distinct extra functions. Supports such as 139,if used, have only the support function. Support 138 provides a conduitfor the high voltage supply connection 130 from unit 32. Support 140 ishollow and provides a suction connection to the aspirating means ofelectrode 136. The supports can conveniently be of plastic tube, toprovide the required electrical insulation and mechanical properties,with the electrode 136 and frame 150 being push-fitted into the tubeends. The junction of connection 130 and electrode 136 is within tube138 in use and is not shown. Frame 150 is also preferably of insulatingmaterial. The supports can be provided with integral or added ribs orother projections to shed liquid from the supports to reduce surfaceleakage via a conductive liquid film.

Portion 144 is hollow to continue the suction connection to tubing 146which extends to the suction part of pump 48. In operation the flow ofliquid through the aspiration means is generally a mixture of air andliquid as bubbles rather than the continuous liquid column of the liquidsupply, so this will cause less electrical leakage than a continuouscolumn and a shorter path is likely to be usable, simplifying thearrangement. A serpentine or coiled path formed by cooperating plasticmouldings may be used to provide the aspiration flow path 146 in a rigidcompact form.

A guard 150 of light, plastics construction can be placed around thesecond electrode supports to prevent casual contact which could give amild electric shock to an operator. The guard must not be so nearelectrode 136 as to interfere with its electrical action on the spray.

The electrode 36 has been described above as being of a round tube, some5 mm in diameter, pierced by holes some 0.5 mm in diameter to enableliquid on the outside to be drawn into the interior of the tube forremoval. While this arrangement is generally effective there areoccasions when the recovery of liquid has to be improved. Also dropletscan form on the outside and upset aspiration and charging.

Other forms of aspirating electrode suitable for use in this equipmentinclude perforated tubes wrapped round with a porous material andelectrodes extended so that aspiration of liquid takes place in a regionwhere the electric field is less intense.

FIG. 4 shows a form of the electrode in FIG. 2 to which these additionshave been applied. The elongate loop electrode is now an unperforatedtube or rod 162 provided with a thin sheet extension 163 on each of theparallel limbs. A perforated tube 164 is provided on sheet 163 to bespaced from the nozzle, such as 20 of FIG. 2, so as to be in a region ofless intense electric field. A layer of porous material 165 is wrappedaround tube 164 to cover the perforations 169. (Layer 165 is shownpartly cutaway.) The porous material may be retained by a clip 166 ofplastics material or other suitable means. Conveniently the porous layer165 is disposable and easily replaced with a new layer, e.g. for adifferent spray liquid. Tube 164 has an extension 167 onto which aflexible tube 168 can be pushed to provide a connection to the suctionconnector of a unit such as 48 of FIG. 1. The perforations 169 arerelatively large compared with those such as 44 in FIGS. 1 and 2. Whentube 164 is 10 mm in diameter, a suitable size, the perforations can be4 mm in diameter. It has been found that small perforations, whencovered by a drop of liquid, are sometimes ineffective in aspirating theliquid. This may be because the surface tension forces of the dropresist aspiration and uncovered adjacent perforations do not create ahigh enough resistance to aspiration of air to ensure that these forcesare overcome. Large perforations would not solve the difficulty but aporous layer will be effective, provided a suitable porosity is used.

A similar sheet 163, tube 164, porous layer 165 and extension 167 areprovided on the other limb. The end of the tube 164 is closed, as shownat 170. In other forms the tube 164 could be looped with a singleextension 167 for the aspiration connection. The materials for theextensions and tubes can be chosen in the same manner as discussed abovefor the electrode itself.

The nature of the porous material must be considered as a hairy oreasily frayed material can provide "whiskers" which act as discharges ofthe electric field. Plastics, foam sponge and some non-woven clotheshave been found suitable.

The arrangements just described can be applied to other forms of nozzlee.g. those producing a circular spray cone. The surfaces of the sheetextensions 163 can help to reduce the number of unwanted large dropletslost from the spray particularly with hand-held use at varying angles asthe operator directs the spray around the plants, trees or crop. Insteadof a tube wrapped with a porous material a length of tube with a porouswall may be used. In either case the porous technique may be applied toelectrodes, as shown in FIGS. 1, 2 and 3, without the extension sheetssuch as 163.

As typical operating conditions are 5 kV to 10 kV at a few tens ofmicroamperes the electric shock receivable is not great, well below theaccepted hazard levels by orders of magnitude. The power requirement isthus about 1 W per spray head. This can be met by dry-cell-poweredoscillator-driven power units for single head portable units and makesquite modest demands on the vehicle electrical supply for a vehicleborne multiple head arrangement. Pump power for a single head is alsonot great. A supply pressure of some 400 KPa (approximately 60 psi) isneeded with a suction of about 10 KPa (say 2 psi) for the aspirating ofthe electrode. For a portable unit the pump requirements might be met byintermittent hand pumping to prime a closed reservoir or by the use of acompressed air bottle charged before connection to the unit. It may bemore convenient to collect the liquid aspirated from the electrode in aseparate vessel than to return it continuously to a closed reservoir.

Arrangements of electrical and liquid supply have been described for asingle spraying assembly which may be for hand-held or vehicle-borneuse, such as that shown in the figures. It will be apparent that thecharging voltage may be applied to any part of a conductive nozzle or atany point in the flow path of a conductive liquid. Consequently amultiple assembly such as would be useful for field spraying from atractor can have either a separate supply to each head or a branchingliquid supply to a group of heads with a single electrical input at orbefore the branching point.

We claim:
 1. An electrostatic spraying apparatus, comprising:a sprayliquid supply means; a spray head having a spray liquid inlet and aspray a first electrode and a second electrode, including meanssupporting said first and second electrode on said apparatus forapplying an electrical potential difference proximally of said sprayexit, whereby emergent spray is inductively electrically charged; saidfirst electrode being associated with said spray head and arranged to bein electrical contact with spray liquid supplied by said spray liquidsupply means to said spray head; said second electrode comprising ahollow body having a wall provided with at least one aperture throughwhich liquid deposited exteriorly on said wall may pass to the interiorof said hollow body; said second electrode being arranged in spacedrelation to said spray exit; and asperating means associated with theinterior of said hollow body for removing liquid deposited on saidsecond electrode by drawing such liquid through said at least oneaperture.
 2. An apparatus according to claim 1, wherein:said sprayliquid supply means includes a spray liquid reservoir and pump means forforcing liquid from said reservoir to said spray head; and saidapparatus further includes means for supporting said apparatus in aspray delivering configuration for directing delivery of charged sprayliquid from said spray exit of said spray head.
 3. An apparatusaccording to claim 1 in which at least part of the hollow body is madeof conductive material, for use with a non-conductive spray liquid. 4.An apparatus according to claim 1 in which the first electrode isconstructed and arranged for use at a potential other than earthpotential and the spray liquid supply means includes a spray liquidcontainer and a long liquid flow path means between the container andspray head liquid inlet to restrict electrical leakage from theelectrode through the liquid flow path means and any liquid therein. 5.An apparatus according to claim 4 in which the long liquid flow pathmeans includes a length of electrically insulating tube.
 6. An apparatusaccording to claim 4 in which the long liquid flow path includes aconvoluted path through a body of insulating material.
 7. An apparatusaccording to claim 4 in which the spray liquid supply means includes anearthed spray liquid container.
 8. An apparatus according to claim 1 inwhich the aspirating means includes aspiration flow path means from theaspirating means electrode to a source of reduced pressure to apply thereduced pressure at said at least one electrode to remove the liquid,the flow path means also being constructed and arranged to conveyremoved liquid over at least part of its length.
 9. An apparatusaccording to claim 8 in which the electrode provided with aspiratingmeans is for use at a potential other than earth potential and theaspiration flow path means is long to reduce electrical leakage from theelectrode through the flow path means and any liquid therein.
 10. Anapparatus according to claim 8 or claim 9 in which the removed liquid isreturned to the spray liquid supply means.
 11. An apparatus according toclaim 8 in which the source of reduced pressure is a liquid jet pumpsuch as an ejector operated by the spray liquid supply means.
 12. Anapparatus according to claim 1 in which the second electrode is at earthpotential.
 13. An apparatus according to claim 12 in which the secondelectrode also forms a guard against contact by an operator withnon-earth potential parts of the apparatus.
 14. An apparatus accordingto claim 1 in which the spray head has a single said spray exit toproduce a spray having one shape of a hollow cone, a solid cone, ahollow fan and a solid fan and the second electrode has the form of aloop conforming to the shape of the spray to preserve a substantiallyuniform spacing between the spray surface and the nearest point of thesecond electrode.
 15. An apparatus according to claim 1 in which thesecond electrode includes at least a portion formed of porous materialfor the receipt of deposited liquid and providing through the pores ofthe material means for the passage of deposited liquid by the action ofthe aspirating means for removal.
 16. An apparatus according to claim 15including in the second electrode support means to support the porousmaterial portion away from the spray exit.